Various metalization, interconnect, and polycide formation processes are known and used in the fabrication of semiconductor devices. For example, in fabrication of static random access memories (SRAMs) or dynamic random access memories (DRAMs), local interconnects are used to increase packing density and metal suicides are used for metalization of contact junctions and for forming polycide lines, such as bit lines, word lines, or other local interconnection of device elements being fabricated. As line widths of semiconductor devices decrease, desire for low resistance polycides with low aspect ratios has led to the consideration of various polycide materials other than the conventional tungsten or titanium silicide. Further, for example, in the formation of shallow junction contacts, a desire for low resistivity materials for metalization of such junction contacts as an alternative to titanium silicide are also desired. As shallower junctions become utilized in, for example, memory device fabrication, the titanium thickness used in a silicidation process, must be reduced, which, in turn, undesirably increases the Kelvin contact resistance. This is because titanium silicide is a large grain material. Thin titanium silicide has nonuniform large grain size that contributes to a very rough titanium silicide/silicon interface. As such, it reduces the effective ohmic contact area. In order to use such lower resistivity materials, effective methods for forming such materials, such as cobalt silicide, need to be developed.
Cobalt silicide, or perhaps nickel silicide, in particular, due to their low bulk resistivity and small grain size, are well suited for use in various fabrication processes, such as, for example, shallow junction contact metalization processes or use in polycide structures, such as polycide word lines or bit lines. However, junction spiking at the cobalt silicide/silicon interface associated with the use of cobalt as the predominant diffusion species in a silicide process, tends to form spiking related pittings, i.e., voids in the cobalt silicide formed. Such pittings or voids are detrimental to the performance of devices fabricated with such a cobalt silicidation process as the pittings may penetrate through the shallow junction to cause junction leakage.
Further, dopants and impurities can diffuse out of cobalt silicide easily. As such, diffusion of impurities out of the cobalt towards the gate oxide of a device may occur. For example, diffusion of impurities, such as alkali metals, may occur in the metalization of a bit line contact during a cobalt silicidation anneal. Such diffusion of unwanted impurities into the gate region may also be detrimental to the devices being fabricated.
Various silicidation methods using cobalt are known. For example, in one such method, a layer of cobalt is deposited over silicon regions and then an anneal is performed to react the cobalt with the silicon to silicide the silicon regions with the formation of cobalt silicide.
In another silicidation method, titanium and then cobalt are deposited on a silicon region and then annealed. During the annealing process, the titanium reacts with the silicon region and then diffuses upward with the cobalt diffusing downward to silicide the silicon regions resulting in the formation of cobalt silicide. This method is known as a titanium/cobalt reversal silicidation process.
As is known and described in the article entitled, "Nitrogen-Doped Nickel Monosilicide Technique for Deep Submicron CMOS Salicide," by T. Ohguro, et al., IEDM (1995), pp. 453-456, the incorporation of small amounts of nitrogen into assputtered nickel film before salicidation can control spiking and improve leakage current. However, nickel, and also cobalt, do not easily react with nitrogen making such incorporation difficult. As such, incorporation of nitrogen into the nickel or cobalt must be performed with an undesirably large volume ratio of nitrogen during the deposition of the cobalt or nickel to incorporate a small amount of nitrogen in the cobalt or nickel silicide film.
However, one shortcoming associated with the nitridation of cobalt or nickel films to decrease junction pittings or voids, is that the difficulty of the silicidation process using such nitridated films increases with the increasing percentage of nitrogen incorporated therein. For example, the temperature of the anneal necessary for obtaining the same resistance for the silicidation process when utilizing cobalt or nickel films having increasing nitrogen concentrations, increases as well.
Accordingly, there is a need in the art for silicidation methods for forming, for example, polycide lines or metalization of contact holes, which overcome difficulties with the use of cobalt and nickel in the silicidation process. Such methods should decrease the potential for pitting during the silicidation process, more efficiently incorporate nitrogen into the cobalt or nickel for use in decreasing such spiking problems, and/or remove impurities of the cobalt or nickel film to decrease the likelihood of diffusion of unwanted impurities therefrom. The present invention overcomes problems described above and other problems as will become apparent to one skilled in the art from the description below.